Receivers are already known that enable the attitude or the pointing direction of a vehicle such as a satellite to be determined by using the signals transmitted by global navigation satellite systems (GNSS), such as the American global positioning system (GPS) and the Russian global orbiting navigation satellite system (GLONASS). Geostationary navigation overlay systems (GNOS) such as the European geostationary navigation overlay system (EGNOS) can also be used.
Receivers are already known that enable the attitude of satellites to be determined from GNSS signals by differential phase measurements, e.g. from the article by V. W. Spinney "Applications of the global positioning system as an attitude reference for near earth uses", published by the Institute of Navigation, Warminster, Pa. USA, April 1976, or from the article by A. K. Brown et al. entitled "Interferometric attitude determination using the global positioning system: a new gyrotheodolyte", published in the proceedings of the 3rd International Geodetic Symposium on Satellite Positioning, held at Las Cruses in the USA in 1982, or indeed from the article by K. M. Joseph et al. entitled "Precision orientation: a new GPS application", published in the report of the International Telemetering Conference held at San Diego, Calif., USA, October 1983.
Determining the attitude of any vehicle (space vehicle, aircraft, ship, etc. . . . ) requires the existence of accurate differential phase measurements of carriers comning from at least two GNSS satellites and received, in general, via a minimum of three non-aligned antennas rigidly mounted on the vehicle. Nevertheless, for two-axis attitude determination, also known as "pointing direction" determination, it is possible to make do with only two antennas.
In addition to knowing simultaneously the position of the receiver and the positions of the transmitters as obtained by tracking and demodulating the GNSS signals, such differential phase measurements allow the receiver to determine the attitude of the vehicle with accuracy that can be better than one minute of arc, under appropriate conditions.
A large amount of research effort has been devoted to minimizing errors relating to attitude determination using GPS, within the constraints arising from the particular architecture used for the receiver.
Two receiver architectures have been developed in detail: parallel architecture, an example of which is given in U.S. Pat. No. 5,185,610 (Texas Instruments); and multiplex architecture as described in U.S. Pat. No. 5,268,695 (Trimble Navigation Ltd.) or indeed in two articles published by Texas Instruments, firstly the article by C. R. Johnson et al. entitled "Applications of a multiplexed GPS user set", published in the Proceedings of the 37th Annual Meeting of the USA Institute of Navigation, 1981, pp. 61 to 77, and secondly the article by R. A. Maher entitled "A comparison of multichannel sequential and multiplex GPS receiver for air navigation", published in Journal of the Institute of Navigation, Vol. 31, No. 2, Summer 1984.
With parallel architecture, measurements are obtained by processing signals via multiple identical channels, each connected to a different antenna. Signals are tracked continuously, thereby obtaining low random noise in the differential carrier phase measurements. However, given that the signals must pass through different analog devices, unknown bias necessarily arises.
In contrast, in multiplex architecture, measurements are obtained using a single channel which is switched periodically between the various antennas. This eliminates the effects of bias, but considerably increases random noise level.
Each of those architectures thus presents drawbacks in terms of results obtained, or in terms of implementation, and these are discussed below.
In parallel architecture as shown in FIG. 1, using four antennas, each of the four antennas is connected to a respective one of four identical processing chains under the control of a digital signal processor (DSP).
In this parallel architecture, the signals received on the various antennas are applied to parallel processing channels, each performing radiofrequency to intermediate frequency (RF/IF) processing, and providing code and carrier tracking loops for at least some of the signals, two being a theoretical minimum, and four GNSS signals generally being processed so as to enable real time position determination. Signals are processed in parallel and independently, but for all the various mixing processes, the reference signals which are applied are derived from the same reference temperature-controlled crystal oscillator TCXO via a frequency synthesizer. If independent reference signals were used, then it would not be possible to perform accurate differential carrier phase measurements, and it would be very difficult to separate real carrier phase differences from the phase drift of each reference signal. In accordance with modern spread-spectrum receiver techniques, Costas loops are used for carrier phase tracking and delay-lock loops (DLL) are used for code phase tracking. On this point, reference can be made in particular to the article by J. J. Spilker entitled "GPS signal structure and performance characteristics", published in the journal Navigation, Vol. 1, 1980.
A parallel architecture receiver has two main drawbacks, namely: hardware complexity; and bias due to propagation along the different lines of the various circuits. Bias is the main cause of loss of accuracy. Each device in the RF/IF chain presents finite group delay and thus gives rise to "line" bias, and in general line bias is not the same for all of the chains. The RF filters and the IF filters contribute not only to the total group delay, but they are also very sensitive to temperature. Although group propagation delays through the RF/IF paths of the different antennas can be matched to some extent and can also be calibrated, mismatching always arises because of temperature variations, radiation, and aging phenomena, which have a direct influence on the accuracy with which attitude is determined. Although on-line recalibration is possible, that significantly increases the complexity of the apparatus.
Finally, the complexity, and thus the cost, of a parallel receiver for determining attitude is comparatively high, given that there must be a separate measurement chain for each antenna. This is a very significant drawback for applications in which volume, mass, and power consumption are of great importance, and in particular on-board a satellites.
Using a multiplex architecture receiver in which the signals output by the various antennas are multiplexed eliminates both problems of line bias and problems of RF interference, because there is only one RF/IF section to which all of the time-multiplexed antenna signals are applied. Multiplexing is performed by means of a fast switch which can easily be implemented with PIN diodes or field-effect transistors. This is shown in FIG. 2.
In a multiplex receiver, the paths of all the signals share a common RF/IF section, such that practically all line bias is eliminated, with the exception of the influence of different lengths of antenna cable. Cross-talk is also eliminated because only one antenna is active at any given time. Multiplex architecture also has the advantage of requiring much less hardware than parallel architecture. This leads to significant savings in terms of volume, mass, power consumption, and cost.
However, antenna-multiplexing receivers also suffer from certain drawbacks. The most significant drawback is represented by loss in tracking performance caused by the fact that the antenna signals are observed only during a certain fraction of time. For example, multiplexing between four antennas can lead to a 6 dB degradation of the signal-to-noise ratio, as shown in the work by J. S. Spilker, entitled "Digital communications by satellite", Prentice Hall, N.J., USA (1977).
In addition, multiplex switching must be performed fast enough to maintain the ability to detect data bit transitions, which take place at 50 Hz in GPS and at 100 Hz in GLONASS. Also, the digital processing circuits must be reinitialized at the beginning of each interval. This implies that the phase of the numerically-controlled oscillator (NCO) used in the code and carrier loops needs to be set to an initial phase value on each occasion, and that the code generator must be initialized in the correct state, thereby greatly complicating the design of the receiver. Furthermore, false lock conditions may occur in the carrier tracking loops as a result of the various sampling and switching rates involved, and they also need to be taken into account.
The basic concept of the present invention is to eliminate, at least for the most part, at least some of the above-specified drawbacks by means of an architecture that combines a continuous receiver chain with a multiplex receiver chain.